This invention relates generally to a direct digital control system, and more particularly to an auxiliary channel operating in conjuction with a selected loop in the plurality of process control loops included in the system to expand the frequency response of the selected loop so that it is responsive to the full range of slow and rapid process variations without interfering in any way with the normal operation of the system.
The expression "direct digital control" or "DDC" as it is commonly abbreviated, refers to the use of a digital computer on a time-shared or multiplexing basis for the control of industrial processes such as those involved in the petroleum and chemical industries. A DDC system therefore includes a plurality of process control loops, each controlling a particular activity. The factor controlled may be flow rate, pressure, viscosity, liquid level, or any other process variable.
In a conventional process control loop, an analog electronic controller acts to determine the deviation of the analog signal generated by a transducer which senses the process variable from a set point and produces a corrective analog output signal that depends on the deviation, the output signal being applied to a final control element correcting the process.
A conventional electronic controller is arranged to operate in a proportional, integral or derivative action mode, or in a combination thereof. Proportional action produces a corrective output signal proportional to the deviation of the process variable signal from the set point. The amount of deviation in terms of percentage required to move the final control element through the full range is known as the proportional band. Integral or reset action produces an output signal that is a function of the length of time controlled process variable has been away from the set point. In derivative or rate action, the resultant corrective signal is proportional to the rate at which the process variable is changing.
In a DDC system, the digital computer therein is common to a plurality of analog process control loops. Because the computer operates on digital data, the analog process variable signal from each transducer must be converted into a corresponding digital value and the computer output must be converted into an analog output signal for operating the associated final control element. Set points are given to the computer from a digital terminal. Also included is a multiplexer, which supplies the respective analog input signals from the several loops to the common computer in sequence, so that the input signals are sampled. Means are also provided to hold the output signals derived from the sampled loops so that no interruption occurs in the signals fed to the associated final control elements.
Typically, the sampling rate in a DDC system is no more than one sample per second. When in a given system each process variable is undergoing change at a relatively slow rate, a sampling rate of one-per-second is generally adequate to cope with these slow changes and to effect corrections therefor through the final control elements.
The concern of the present invention is with regard to those process control loops in a DDC system which respond to a process variable subject to change at a relatively rapid rate. When the sampling speed is one-per-second, then the corner or break frequency of the loop may be lower than 0.5 Hz; and if the process variable is undergoing change at a relatively rapid rate, the loop will then be incapable of adequately responding to the process variable to effect the necessary correction.
One obvious solution to this problem is to design a DDC system with a much higher sampling speed; but to do so would unduly complicate the circuit design of the system and add substantially to the cost thereof.
Another approach to the problem is to provide separate analog electronic controllers only for those loops requiring a high-frequency response, the other loops which are responsive to low-frequency process variables lying within the system. Thus if there are sixteen loops to be controlled, and five of these require high frequency operation, then eleven loops will be included in the DDC system and five will be operated through individual analog process controllers. This hybrid system is not only a relatively costly solution to the problem, but it also has operational disadvantages, for the operator would have to run the overall system from more than one place; for he would have to operate the DDC system from its local control terminal and at the same time operate the analog controllers from their respective front panels.